The liquid crystal display devices are used in both direct view display and projection display for numerous information display purposes.
Generally, the display modes include, among others, phase change (PC), twist nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), and vertical alignment (VA).
The devices operating in TN and STN modes employ positive dielectric anisotropy liquid crystal, the devices operating in ECB and VA modes employ negative dielectric anisotropy liquid crystal, while those operating in IPS mode may employ both positive and negative dielectric anisotropy liquid crystals.
The display devices may be categorized into passive matrix (PM) and active matrix (AM) according to the driving mode. The latter utilizes thin film transistor (TFT), metal insulator metal (MIM), etc.
Recently, the liquid crystal displays with features such as versatility, large screen, high definition and fast response are more and more popular. Such displays have to be driven in active matrix mode, in which TFTs are used mostly. In an AM-TFT device, the liquid crystal material, as the medium, and the liquid crystal cell constitute a capacitor. But in practice, the capacitor can't hold the voltage till the next refresh frame. To address this, each pixel is provided with a field effect transistor (FET). The pixel electrode is being charged when the TFT is on, while the capacitor is being charged when the TFT is off, until at next cycle the pixel is addressed again. The electric discharge rate of the pixel depends on the capacity of the electrodes and the specific resistance of the dielectric material therebetween. Therefore, a liquid crystal material with high specific resistance and suitable dielectric constant is desirable.
Being provided with a liquid crystal material with high specific resistance, the liquid crystal display device may have increased voltage holding ratio, and as a result, increased contrast ratio. Therefore, it's desirable for the liquid crystal material to have large specific resistance initially and retain a reasonable resistance after prolonged usage.
Being provided with a liquid crystal material with suitable optical anisotropy, the liquid crystal display device may have increased contrast ratio.
Being provided with a liquid crystal material with small viscosity, the liquid crystal display device may have short response time. With a short response time, the liquid crystal display device is suitable for displaying animations. Furthermore, a small viscosity is conducive to the injection of the liquid crystal material into the liquid crystal cell, thereby shortening the injection duration and further improving the operability.
Therefore, it is a goal for those skilled in the art to optimize the liquid crystal material to obtain increased specific resistance, increased voltage holding ratio, suitable optical anisotropy, small viscosity, low threshold voltage and large dielectric anisotropy.
The prior art, such as CN102858918A, teaches a liquid crystal composition with high voltage holding ratio, low power consumption and short response time, however, the prior art is environmentally unfriendly (due to the usage of chlorine compounds) and fails to achieve a balance among such properties as a high specific resistance, low-temperature storage stability, suitable optical anisotropy, high dielectric anisotropy, short response time, low driving voltage and small viscosity required by the liquid crystal television set, flat panel TV and the like.
From the perspective of manufacture, the respective properties of the liquid crystal material are mutually influenced by each other, that is, the increase of certain property may incur changes to other properties. Therefore, it requires inventive efforts to prepare a liquid crystal material with suitable properties on all aspects.
The liquid crystal material is an important component of the liquid crystal display. There are huge demand in the global market for the liquid crystal displays, which are used primarily in the electronic products but suffer from a short operating life. A short operating life implies the problem of waste and pollution, which is more and more unacceptable with the environment-friendliness receiving increasing attention of the public. It would greatly reduce the cost of the disposal of the used liquid crystal displays to select environmentally friendly constituents in preparing the liquid crystal material. Therefore, it requires inventive efforts to prepare a liquid crystal material not only having suitable properties on all aspects, but also being economically feasible and environmentally friendly.
A purpose of the invention is to provide a liquid crystal composition with at least one of the following properties: a large specific resistance, a high voltage holding ratio, a high clear point, a suitable optical anisotropy, a suitable dielectric anisotropy, a small viscosity, a low threshold voltage, etc. To achieve the environmental friendliness, the invention forgoes the chlorine monomer for difluoroether monomers. The difluoroether monomers more than make up for the drawbacks such as elevated threshold voltage, lowered clear point and optical anisotropy, and enable faster response and improved voltage holding ratio relative to ordinary medium-polarity monomers.
Another purpose of the invention is to provide a liquid crystal display device employing a composition with at least one of the following properties: a large specific resistance, a high voltage holding ratio, a high clear point, a suitable optical anisotropy, a suitable dielectric anisotropy, a small viscosity, a low threshold voltage, etc., and having characteristics such as a high voltage holding ratio, a short response time, a high contrast ratio, a low power consumption and environmental friendliness.